Theodor Mommsen is considered as one of the most famous classical scholars, historians and jurists. With his book on roman public law he has until now a days a influence on study of the law of the roman republic. Nevertheless Mommsen himself feeld insatisfied with his results. The present contributions states, that this discontentedness on his own work has to be seen in context with the methodical approach chosen by Mommsen. The interpretation he gave on the roman republic was very much influenced by the liberal thinking of his own time. We examine the signification of the word „Willkür” (arbitrariness) in Mommsens work on the roman public law. The examination of this notion offers a possibility to see, how Mommsen structured the sources of roman law in order to construct the system of the roman public law created by himself.